In the prior art various sensors have been developed for the purpose of detecting a collision situation in a motor car at the earliest possible moment in time. Such sensors have been installed within the so-called zone of deformation of a motor vehicle. The sensors are adapted to detect how intense the collision is and trigger safety installations on board of the vehicle, for example safety belt tensioning installations or adaptive multi-stage airbag systems. Prior art sensors are mostly fully-electronic acceleration or deceleration sensors located in the front portion of a vehicle. Such sensors are, for example, located within the suspension of the cooling system, within the chassis or within the shock absorbers. Within the particular art, these sensors are also denominated as “up-front sensors”. These sensors generate an output signal corresponding to the particular collision. The output signal is transferred to a central processing unit via an appropriate cable.
Within the central processing unit the output signal is further processed. Conventionally, this is effected with the help of mathematical algorithms according to which the signal is integrated over the collision period of time. From the velocity change resulting therefrom, criteria are derived for triggering the various safety installations.
Considering that in practice collisions of motor vehicles happen under highly varying conditions, the acceleration peak values at the location of the sensors may assume highly different values.
In practice, a dynamic range of about 200:1 within a frequency band of between 50 Hz and 5,000 Hz has to be taken into account. In view of these figures it becomes clear that the limited dynamic range of conventional sensors will result in signal distortions which, in turn, result in a deterioration of the above-mentioned time integral, so that the safety criteria are subjected to an increased and unwanted fuzziness.
German patent specification 41 28 347 C1 discloses an impact-induced pulse sensor which does not operate according to the afore-mentioned principles, i.e. does not derive an analog measuring signal from a physical quantity (acceleration, deceleration). Instead, this prior art sensor is structurally configured as a safety switch or, as identified in the art, a so-called “safing sensor”. This prior art sensor simply comprises a conventional spring-mass-system in which an annular magnet is arranged to slide within a tube and may be displaced against the force of a compression spring. When the annular magnet passes by a magnetically operable electrical switch, for example a Reed contact, a measuring signal is generated, namely the switching signal that appears when the Reed contact is closed or opened, respectively. The displaceable magnet is guided with a lateral pin thereon engaging a groove extending along the trajectory of the magnet and being inclined relative thereto. Due to this configuration, a rotational movement is superimposed to the axial movement of the magnet. The configuration of the groove is selected such that the magnet is only slightly decelerated when advancing to an end position, but is strongly decelerated when returning back.
This measure has been taken in order to make the closed period of time of the safety switch highly independent from the particular shape of the collision graph for achieving an overall longer closed period of time.
This prior art sensor, therefore, does not generate an analog measuring signal that would be adapted to be processed further. Therefore, this prior art sensor belongs to an entirely different type of sensors as compared to the one specified at the outset.
In these prior art sensors, the pulse velocity of the displaced sensor element depends on the amplitude and on the duration of the impact acting on the sensor. If measuring results with impact-induced pulses of different amplitude/duration are compared, which, however, have like pulse areas, these prior art sensors yield differing measuring signals.
U.S. Pat. No. 5,983,724 discloses an impact-induced pulse sensor in which the sensor element comprises a rotor being journalled excentrically and having two magnets at its periphery. These magnets are retained in a predetermined initial position by means of stationary retention magnets being arranged at a distance. In this initial position, the two rotor magnets are located vis-à-vis a respective sensor element. The sensor elements comprise a resistor bridge circuit with magnetoresistive elements.
If an impact or shock is exerted on this prior art sensor within a plane arranged perpendicular to the rotational axis of the rotor, the rotor overcomes the retention force of the retention magnets and departs from its initial rest position. Thereby, the magnets move relatively to the sensor elements and the bridge circuits therein are detuned accordingly. The signal thus generated does not linearly depend on the prevailing acceleration. Therefore, linearizing circuits are provided for generating a linear dependency of the sensor signal from the prevailing acceleration. By means of a comparison with a nominal value, a threshold is defined and, if the threshold is exceeded, a triggering signal is generated by the impact-induced pulse sensor.
Therefore, in this prior art sensor no measuring voltage that would depend from the particular distance of the sensor element is processed and, hence, no independence from the pulse velocity may be achieved.
U.S. Pat. Nos. 5,756,896 and 5,149,925 disclose impact-induced pulse sensors utilizing a linearly moved permanent magnet as a sensor element. U.S. Pat. No. 3,483,759 discloses a velocity transducer comprising a cylindrical rod magnet suspended by a spring within a tube having a winding positioned thereon in such a manner that the movement of the magnet within the tube induces a voltage in the winding proportional to the impact velocity of a shock experienced by the transducer.
It is, therefore, an object underlying the invention to provide an impact-induced pulse sensor of the type specified at the outset such that the above-mentioned disadvantages are avoided. In particular, a sensor output signal shall be generated that is a direct measure for the pulse velocity, i.e. is independent from the duration of the prevailing pulse-shaped impact for like pulse areas of the impact.